Expression of the glucagon gene yields a tissue-determined variety of peptide products that are processed from the 160 residue proglucagon product. The organization of these peptides within the proglucagon precursor was elucidated by the molecular cloning of preproglucagon cDNAs from the rat, hamster and bovine pancreas. These analyses revealed that preproglucagon contains not only the sequence of glucagon and glicentin, but also two additional glucagon-like peptides (GLP-1 and GLP-2) separated from glucagon and each other by two spacer or intervening peptides (IP-I and IP-II). These peptides are flanked by pairs of basic amino acids, characteristic of classic prohormone cleavage sites, suggesting they might be liberated after posttranslational processing of proglucagon (Drucker, Pancreas, V1990, 5(4):484).
Analysis of the peptides liberated from proglucagon in the pancreatic islets of Langerhans, for instance, suggests the primary pancreatic peptide liberated is the 29-mer glucagon, whereas glicentin, oxyntomodulin, IP-II and the glucagon-like peptides are more prevalent in the small and large intestines. This demonstration that the glucagon-like peptides are found in the bowel has prompted research into the precise structure and putative function(s) of these newly discovered gut peptides. Most studies have focussed on GLP-1, because several lines of evidence suggested that GLP-1 may be an important new regulatory peptide. Indeed, it has been determined that GLP-1 is one of the most potent known peptidergic stimulus for insulin release, an action mediated in a glucose-dependent manner through interaction with receptors on pancreatic .beta. cells. GLP-1 and its derivatives are in development for use in the treatment of diabetics.
The physiological roles of glicentin and oxyntomodulin, the so-called "enteroglucagons", are also under investigation, particularly with respect to regulation of acid secretion and the growth of intestinal cells.
Oxyntomodulin is capable of inhibiting pentagastrin-stimulated gastric acid secretion in a dose-dependent manner. The role of glicentin in mediating the changes of intestinal adaptation and growth of the intestinal mucosa has been investigated, and the intestinotrophic effect of glicentin and its therapeutic use have recently been reported by Matsuno et al in EP 612,531 published Aug. 31, 1994.
In contrast to GLP-1 and other glucagon-related peptides, the physiological role of glucagon-like peptide GLP-2 remains poorly understood despite the isolation and sequencing of various GLP-2 homologues including human, rat, bovine, porcine, guinea pig and hamster. Using GLP-2 antisera raised against synthetic versions of GLP-2, various groups have determined that GLP-2 is present primarily in intestinal rather than pancreatic extracts (see Mojsov et al, J. Biol. Chem., 1986, 261(25):11880; Orskov et al in Endocrinology, 1986, 119(4):1467 and in Diabetologia, 1987, 30:874 and in FEBS Letters, 1989, 247(2):193; George et al, FEBS Letters, 1985, 192(2):275). With respect to its biological role, Hoosein et al report (FEBS Letters, 1984, 178(l):83) that GLP-2 neither competes with glucagon for binding to rat liver and brain tissues, nor stimulates adenylate cyclase production in liver plasma membranes, but, enigmatically, can stimulate adenylate cyclase in both rat hyopthalamic and pituitary tissue, at 30-50 pM concentrations. An elucidation of the physiological role of GLP-2 would clearly be desirable.